I cannot imagine any DB pension scheme would try and prevent anyone aged over 55 from taking a pension. The last thing any principle employer of a DB wants is uncertainty as to future liabilities, by taking an early pension these liabilities can be crystallised and funded. Almost certainly the Scheme rules will treat anyone taking an early pension as being 'Scheme neutral' for the finances of the Scheme, in other words you can go early and get a smaller pension for more years, or go later and get a larger pension for fewer years - but the amount paid out by the scheme* will be the same - hence the term 'Scheme neutral'. I would be truly very surprised if the BA Scheme(s) did not include this as its fairly common practice in 'big' schemes.
* taking predicted inflation and pension increases into account, the Scheme only 'loses' if you like if you live longer than the expected likely mortality rate for your particular cohort.